This application claims priority to Japanese Patent Application Number JP2002-054837 filed Feb. 28, 2002, which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to an exposure apparatus and an exposure method for exposing a negative by making use of near-field light, and a recording and/or reproducing apparatus and a recording and/or reproducing method primarily for recording information signals in an optical recording medium by making use of near-field light.
2. Description of the Related Art
Recently, in response to the demand for a higher density of an optical disk, a solid immersion lens (SIL), which is shaped like a spherical lens partly cut off and exhibits a high refractive index, is disposed between an object lens and a negative or an optical disk thereby to achieve a further smaller spot diameter. Thus, a larger numerical aperture (e.g., larger than 1) NA than the numerical aperture NA of the object lens itself is achieved. The SIL is disposed such that its spherical surface faces the object lens, while the opposite surface, namely, its cut surface, faces the negative or the like. The larger numerical aperture can be achieved also by using a solid immersion mirror (SIM).
To perform exposure of a negative or to record or reproduce information signal by using the SIL, the laser beams that have passed through an object lens are condensed at the SIL and the distance between a distal end surface, i.e., the lowermost edge surface, of the SIL (the surface of the STL that opposes the negative or the like) and the surface of the negative or the like to be irradiated is shortened to be close to a near-field region where near-field light is generated. Furthermore, gap control has to be carried out to maintain a constant distance or gap length between the distal end surface of the SIL and the surface of the negative or the like to be irradiated.
Carrying out the gap control requires the detection of the gap length. One of the available methods for such detection is a total reflection detection method. The total reflection detection method utilizes the fact that, if the distal end surface of the SIL is considerably spaced away from the surface of the negative to be irradiated, then the high NA components (NA≧1.0) of the light incident upon a converging lens assembly formed of the SIL and an object lens are totally reflected by the distal end surface of the SIL, while the optical power of return light reduces as the distal end surface of the SIL approaches a near-field region with respect to the surface of the negative to be irradiated.
In other words, servo control is conducted to control the position of the converging lens assembly so as to maintain the gap length at which the reduced optical power of the light returning from the distal end surface of the SIL reaches a predetermined value. This is the gap control whereby the distance between the distal end surface of the SIL and the surface of the negative to be irradiated is maintained at a constant value.
However, when the light reflection used for negative exposure or for recording or reproducing information signals in order to carry out the gap control based on the total reflection detection method, the quantity of light incident upon the SIL, which should provide a reference, varies according to exposure or recording conditions or the like. This means that the gap length varies accordingly unless, for example, the servo retracting target value is readjusted each time exposure power or the like is changed.
Furthermore, if the irradiation power required for negative exposure or recording or reproducing information signals is small, then the signals may be overpowered by noises even when the total reflection detection described above is performed, providing no adequate servo detection signals.
In addition, if an exposure beam is modulated on the basis of a recording signal, then the modulated signal will be undesirably superimposed on a detection signal used for the servo control. Especially if a servo zone and a modulated signal zone are significantly away from each other, it is difficult to completely remove modulated signal components by using a frequency filter, although it is possible to reduce the modulated signal components. This results in noises.
As a solution to the problem described above, there has been proposed to use light dedicated to gap control that is not involved in negative exposure or the recording or reproduction of information signals. The dedicated light is separate from the light for exposure or the like, and has a constant quantity of light of adequate optical power.
However, using the gap control light independently from the exposure light or the like inevitably causes chromatic aberration of a converging lens assembly formed of an object lens and the SIL because of the difference in wavelength between the gap control light and the exposure light or the like. Hence, it is necessary to correct the chromatic aberration. As the wavelength difference between the gap control light and the exposure light increases, it becomes more difficult to correct the chromatic aberration. It is desired, therefore, to carry out the gap control under a condition where the chromatic aberration is minimized.